Metallurgical applications of the 21/2D TEM technique
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THE microstructure of interesting, contemporary alloys is often highly complex on a microscopic scale. For instance a small-grain matrix structure may typically be combined with a number of fine precipitate phases. Identification of important features, which influence the bulk properties of the material, is then a difficult task. Transmission electron microscopy (TEM) is usually employed, through conventional procedures such as bright and dark field imaging and analysis of selected area electron diffraction patterns. However, a major problem with this approach concerns the unambiguous characterization of the microstructural components. The diffraction pattern contains information about the lattice spacings and orientation of the crystalline phases present. Association of diffraction spots (reflections) with the appropriate image features is achieved by dark field imaging. In a complex alloy, the diffraction patterns contain spots so closely spaced, that it is impossible to form separate images from each reflection using even the smallest feasible objective apertures. Phase identification, by correlation of a feature with its diffraction spot, thus requires analysis of a tedious series of careful microscopy experiments--or may not even be possible. An alternative imaging method has recently been introduced, the major advantage of which permits such characterization in a straight-forward manner. The technique, called 2 VEDimaging, ~involves taking two dark field pictures at different objective lens focus. When viewing the images as a stereo pair the height of a feature is directly related to the position of its associated reflection in reciprocal space. The purpose of this article is to illustrate the usefulness of the 2 V2Dmethod, particularly for metallurgical applications where it is largely unknown. Aspects of the technique which have become evident during the present investigation are also emphasized. R. SINCLAIR is Associate Professor and T. YAMASHITA is Research Assistant, Department of Materials Science and Engineering at Stanford University, Stanford, CA 94305. G. M. MICHAL is Research Metallurgist for the Republic Steel Corporation Research Center, 6801 Brecksville Road, Independence, OH 4413 I. Manuscript submitted February 6, 1981. METALLURGICAL TRANSACTIONS A
1. BACKGROUND The 2 l/2D technique was introduced by Bell? who has reviewed the associated theory and its range of application. 1Only a limited number of examples of its use have appeared in the open literature. L3,4In this section, the principles of the 2 I/zD method are outlined in a way which is intended to complement the details given by Bell. 1 Adjustment of the objective lens (focus) control of a TEM results in the movement of image features, an effect which is most noticeable when focussing the image in the absence of an objective aperture. The degree of movement is related to the angle (a) the imaging beam travels with respect to the microscope optical axis. The origin of this phenomenon may be seen from Fig. 1. The image shift (y) is given
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